Modem cellular communication systems, such as GSM, the IRIDIUM.RTM. system and others, use subscriber identity module (SIM) cards to personalize radio units into which the SIM cards are installed. The personality given to a radio unit by a SIM card makes that radio unit serve the needs of the subscriber who owns the SIM card. Conventional SIM cards are smart cards which include subscriber-specific data and a processing capability. The data may define an international subscriber directory number (ISDN) or phone number, a subscriber's memorized speed dialing numbers, the identity of the subscriber, specific radio unit features to which the subscriber has subscribed, and a cryptographic security key.
Cellular communication systems require SIM cards in order to deliver communication services. For example, radio units are informed of incoming or terminating calls by ring alert or paging radio signals that convey ISDNs or TMSIs (temporary mobile subscriber identities). The presence and use of a SIM card is conventionally required in a particular radio unit so the radio unit will know which ISDN represents an incoming call to that particular radio unit.
The SIM card is also conventionally required as an integral part of system security. An authentication process is performed during registration and prior to servicing incoming and outgoing calls. Only if the authentication process is successful will the registration or call be allowed to take place. Typically, the system initiates the authentication process by issuing a random number to the SIM card being tested for authenticity. The processor on the SIM card then uses this random number, its internal cryptographic security key, and an encryption process performed by the SIM card to generate a response. The SIM response is sent back to the cellular system where it is tested for validity. If the cellular system determines that the SIM response is correct, the SIM card is declared valid.
To maintain system security, the SIM card is constructed so that its cryptographic security key cannot be read from or otherwise divulged by the SIM card. Because of this, a SIM card cannot be easily duplicated. Thus, the SIM card allows the system to provide authentication services in addition to communication services. The authentication services reduce the theft of communication services by unauthorized persons, allowing legitimate subscribers to enjoy reduced costs and better service quality as a result. The SIM card is a valuable item because it provides the key which unlocks communication services for which the subscriber will be billed. However, if the SIM card becomes lost or stolen, it may be fraudulently used to steal communication services using almost any radio unit.
As a consequence of the integration of SIM cards into the operation of cellular systems, a one-to-one correspondence between a SIM card and a radio unit conventionally results. This is because the SIM card is conventionally required to register and to make incoming and outgoing calls, and a radio unit can do very little without the SIM card being present.
Unfortunately, this one-to-one correspondence poses a problem for certain applications, such as public phone exchanges. One such application is a public phone exchange located in an aircraft. In an aircraft a subscriber's cellular phone may be unusable because the aircraft body may severely attenuate radio signals. A public exchange may be usable because an antenna for such an exchange may be located outside the aircraft body. Moreover, it would be desirable for a subscriber to use his or her SIM card with the public exchange so that billing records are kept straight and so that incoming calls may be directed to the user's ISDN. However, the one-to-one correspondence between SIM cards and transceivers would require a radio unit or transceiver for each subscriber wishing to take advantage of such a public phone exchange. Moreover, a public exchange which uses subscriber-provided SIM cards risks weakening security for the system and its subscribers because the SIM cards would be used in a public rather than private phone.
Accordingly, a need exists for a public phone exchange and method which extends the authentication services provided by the use of SIM cards without significantly weakening security and without requiring the inefficiency of a one-to-one correspondence between SIM cards and transceivers.